D.-c. biased scr switch



E. H. HOCIDPER ETAL 3,532,902-

Oct. 6, 1970 .-C. BIASED-SCR SWITCH Filed April 10, 1968 2 Sheets-Sheet1 mvmon EDWARD H. HOOPER, FREDERICK a. mu. MEYER JFPOF- kmd MOE.

SAUL M. BE/SER.

. ATTORNEYS Oct- 1970 E. H. HOOI =ER ETAL .3,532,9 0' 2 I D.-C. BIASEDSCR S W ITCH Filed April 10, 1968 Y 2 sheezs sheet 2 l scn l 29 was:

299 ROVIBAR sch fit/22" LJE'ILYEJ .H-- 40 AMPS i U 40 AMPS I sch 5 AMPS5 AMPS mvamox FREDERICK a. mu. MEYER,

EDWARD H. HOOPER saw. M. firs/s51? ATTORNEYS United States Patent Oflice3,532,902 Patented Oct. 6, 1970 US. Cl. 307252 7 Claims ABSTRACT OF THEDISCLOSURE A direct current (VLF) switch using silicon controlledrectifiers at their power frequency rating. SCRs selectively short-outturns in an antenna tuning coil to bring about a change in inductance. Aturn-on signal is required to close the SCR switch, a turn-off signal toopen the SCR switch and a bias supply to maintain the SCR switch in aclosed state.

BACKGROUND OF THE INVENTION The present invention relates generally toswitching circuits and more particularly to switching. circuitsemploying SCRs to selectively tune resonant circuits.

The Navy has the communications requirement of keeping in contact withships and submarines at sea, at great distances up to several thousandsof miles. Reception must be possible 24 hours a day. To achieve thiscommunication to fleets and task forces with the greatest reliability,high power, low frequency (LF) and very low frequency (VLF) transmittersare used. Transmitters which use up to 1 megawatt of power atfrequencies of from 15 kHz. to 30 kHz. are strategically located aroundthe world. The transmitting antennas used on the above transmitters,being electrically short in physical size, inherently have a very highQ. This seriously limits the (CW) keying speed and when frequency shiftkeying is used, the transmitters suffer because of the severedistortions produced by the limited bandwidth of the antenna resonantcircuit.

The problem of adapting high-speed communications to high power (VLF)transmitters can be solved by having the resonant antenna circuits tunedat all times synchronously with the instantaneous driving frequency.Heretofore, this has been attempted by the use of large saturablereactors which change their reactance to compensate for the change infrequency by means of a D.-C. control voltage. These reactors are slowin action, inefficient, large in size, very costly, and need relativelylarge amounts of D.-C. power. The saturable reactor works on theprinciple of varying the D.-C. reactor control current in accordancewith the teletype input signal. The (RF) winding of the reactor isconnected in parallel with part of the antenna loading coils. As theD.-C. varies, so does the inductance of the reactor, which in turn,changes the resonant frequency of the antenna at the teletype keyingrate. The transmitter output frequency is synchronized with the changesof the antenna since the keying voltage is obtained from the reactorcontrol circuit. The voltage developed across a low value resistor (0.04ohm, for example) in series with the reactor control circuit is fed backto the oscillator. 7

With the use of SCRs to modulate the antenna tuner, switching speeds arealmost instantaneous, i.e., in microseconds; efiiciency is considerablyhigher; they can be easily packaged in smaller units; are relativelyinexpensive; and do not require great amounts of D.-C. power.

When SCRs are used to control or switch signals at frequencies muchabove conventional power frequencies,

they must be derated because of limits on di/dt imposed by turn-onphenomena. For example, an SCR is characterized by turn-on over arelatively small fraction of the total junction upon gating. Spreadingof the on area to the total available on area requires up to 40 ormicroseconds for high current SCRs. As a result, when such SCRs are usedto switch signals of 30 kHz. or therabouts (33 microsecond period) thecomplete junction is never used and advantage cannot be taken of thefull device rating.

SUMMARY The general purpose of this invention is to provide a circuitconfiguration for employing SCRs at their power frequency ratings tocontrol signals at very low frequencies (up through 30 kHz.). Diodes areadded in series with each of the SCRs, thus providing a circuit whichlends itself to D.-C. biasing of the SCRs without changing the externalcircuit. DC. current is caused to flow through the SCRs from an externallow voltage D.-C. source to keep the SCRs on when once turned on. Toturn the switch olf, the D.-C. bias source is simply opened. Once theSCRs are turned on and the bias loop closed, the SCRs can be maintainedfully on by the bias along independently of external circuit conditions.

An object of the present invention is to provide a new and improvedbistable switch.

Another object of the invention is to provide a new and improvedbistable switch employing D.-C. biased silicon-controlled rectifiers.

Still another object of the invention is to provide a new and improvedbistable switch that is capable of switching at high speed and whichutilizes a minimum number of components.

A further object of the invention is to employ SCRs at their powerfrequency ratings to control signals at (VLF) frequencies.

A still further object is to provide switching circuits employing SCRsto selectively tune resonant circuits.

Still another object of the invention is to provide switching circuitsemploying SCRs to modulate an antenna tuning circuit by sequentiallyshorting or opening coils.

BRIEF DESCRIPTION OF THE DRAWINGS With these and other objects in view,as 'will hereinafter more fully appear, and which *will be moreparticularly pointed out in the appended claims, reference is now madeto the following description taken in connection with the accompanyingdrawings in which:

FIG. 1 is a schematic diagram of a prior art device and FIG. 2 is agraphical representation useful in understanding the prior art.

FIG. 3 is a schematic diagram of a switching circuit, and FIG. 4 is agraphical representation useful in understanding, one embodiment of thepresent invention.

FIG. 5 is a schematic diagram of a switching circuit including atriggering circuit, and FIG. 6 is a graphical representation useful inunderstanding one embodiment of the invention.

FIG. 7 is a schematic diagram of a preferred embodiment of the presentinvention.

INTRODUCTION (VLF) antennas, although very large physically, are veryshort electrically. They display an input impedance which is highlycapacitive. In practice, the antenna capacitance is tuned out with alarge coil, and as a result of this combination, the transmitter sees avery high Q-series resonant circuit as a load. The Q is sufiicientlyhigh that even the 40 or .50-cycle shift in transmitter frequencyemployed in frequency shift keying techniques results in a seriouschange in antenna system impedance. To overcome this difliculty,circuits have been developed for shifting the tuning of the antenna withshifts in transmitter frequency by changing the inductance of the tuningcoil.

The requirements imposed by the antenna tuner on the switch are severefor SCRs. SCR ratings were considered in the light of the prior artcircuit configuration shown in FIG. 1. The technique employed is tocontrol the inductance of a large coil 11, used to tune the antenna byshorting out turns of the coil 11 with electrically controllableswitches depicted as silicon controlled rectifiers SCR 12 and SCR 13.

In this explanation, the term current is used to describe an energy flowfrom a first plane of potential toward a second plane of potential,where the first plane is of a polarity positive with respect to that ofthe second plane. In that the operation of a silicon controlledrectifier is much like a latching switch, the terms conducting andnonconducting, open and closed, or on and off are used to describe thestates or conditions of these switches in the following explanation.

As shown in FIG. 1, the SCRs 12 and 13 are gated on alternate halfcycles, conduct for a half cycle and then turn off for a half cycle.FIG. 2 is a graphical representation of the current flow in the circuitof FIG. 1. In such use at 30 kc., each SCR is gated and only particallyturns on in the 16 microseconds that it conducts before voltage isreversed on it and it turns off. In such use, only a portion of junctionis used and the SCR must be derated accordingly. In addition, furtherderating is required because of the effects of passing through thehighly dissipative initial turn-on period 30,000 times per second. Forexample, a 110 amp. r.m.s. SCR must be derated to about amp r.m.s. forsuch use.

The present invention provides a circuit configuration for utilizingSCRs at their power frequency ratings to control signals at very lowfrequencies. Any attempt to reduce the number of switch sectionsrequired by using the highest voltage, highest current SCRs availablewas restricted considerably by dv/dt and recovery time characteristicsof the SCRs. At kc., the dv/a't which the switch must support is (0.19 Xv volts/microsecond in the steady state, or about twice this in thetransient state. A realistic dv/dt rating is volts/microsecond whenturn-on and recovery time requirements are considered. SCRs with higherdv/dt ratings are available; however, some sacrifice in other ratingsmust be made. This imposes a peak voltage limit on the SCR of about100/.38=270 volts. On this basis, it was decided that without using SCRsin series, the voltage rating of the switch should not exceed 200 voltsr.m.s. Considering the double voltage transient, SCRs rated at 600 voltsmust be employed (200 X /2 X 2:566 volts).

The use of high current SCRs is hampered by re covery time. One methodof meeting the recovery times required by 30 kc. operation is to use anSCR selected for this purpose. One of the device characteristics whichaccompanies fast recovery appears to be high forward voltage drop duringconduction, which means lower current rating because of junctionheating. It was concluded that the SCR to be used would be a 600-voltdevice with a power frequency r.m.s. current rating in the range of 55to amps. In a directly operated switch configuration, the switch ratingfor a pair of SCRs would be about 8 kva. With some means of keeping theSCRs on during their nonconducting half cycle, this switch can beincreased to 20 kva.

DESCRIPTION OF THE PREFERRED EMBODIMENT From inspection of FIG. 3, it isapparent that an (RF) switch circuit in accordance with the invention ineffect comprises three subcircuits. Input conductor 27 is coupled to adiode 19 at the cathode 19c. The diode .19

4 has an anode 19a coupled through conductor 28 to the cathode of SCR17. Anode 17a of SCR 17 is connected to input terminal 16 throughconductor 26. Current flow in this first subcircuit is designated as Iin FIGS. 3 and 4.

Input conductor 27 is also connected to anode 21a of diode 21. Cathode21c of diode 21 is connected through conductor 24 to anode 18a of SCR.18. Cathode of SCR 18 is connected to input terminal 16 throughconductor 26. Current fiow in this second subcircuit is designated I inFIGS. 3 and 4.

The bias circuit I begins with an outside voltage source 22 having apositive terminal connected to one end of current limiting resistor 23which has its other end connected to fixed contact of a so-calledmechanical make and break switch 14. The other fixed contact of a switch14 is connected to cathode 21c of a diode 21 and to anode 18a of an SCR18 through conductor 24. Cathode 180 of SCR 18 is connected to terminal16 and to anode 17a of SCR 17 through conductor 26. Cathode 170 of SCR17 is connected to the negative side of an outside voltage source 22 andto the anode 19a of diode 19. Bias current flow in this third subcircuitis designated I in FIGS. 3 and 4.

Operation of an embodiment of the RF switch circuit of the invention asshown in FIG. 3 is as follows. With the switch 14 closed before the SCRs17 and 18 are gated, the voltage across SCRs 17 and 18 is the externalterminal voltage across terminals 15 and 16 shared with the diodes 19and 21 plus a portion of the bias voltage E. When SCRs 17 and 18 aretriggered on, either simultaneously or synchronously with the externalbias voltage, SCRs 17 and 18 turn on or enter a conduction state and arekept there by current flowing from the bias source 22 through a currentlimiting resistor 23. On data pulses, a series of square wavessynchronous with the antenna current, are obtained from an on driver(not shown) and fed to gates 17g and 18g of SCRs 17 and 18. Once theSCRs 17 and 18 are triggered on and bias current I begins to flow, theSCRs 17 and 18 remain on regardless of the voltage on the externalterminals 15 and 16.

The diodes 19 and 21 in the circuit support any internal back voltageswhich would act to turn the SCRs 17 and 18 off. D.-C. bias current Iflows in and is confined by the diodes 19 and 21 to the subcircuit loopas indicated by 1 External circuit current flows in first subcircuitpath I or in second subcircuit path I depending upon the polarity of anexternal signal.

To turn the (RF) switch circuit off, bias current 1 is interrupted byopening switch 14. Because of opposite polarities of the externalsignal, external circuit current flow can fiow through only one SCR atany time. If at the time of opening switch 14, external current isflowing in path I SCR 18 Will turn off, leaving SCR 17 conducting. Whenthe external current returns to zero SCR 17 will turn off. Since SCR 18is already otf, current cannot reverse. After a few microseconds toallow the SCRs 17 and 18 to fully recover, switch 14 may be closed toarm the (RF) switch circuit for turn on by triggering when required.

An alternative embodiment to closing switch 14 is shown in FIG. 5 wherea resonant crowbar arrangement may be employed on the bias source 22'.With the (RF) switch circuit of the invention on and the bias source 22supplying bias current I as shown in FIG. 6, turn off is initiated bytriggering the crowbar SCR 29 by applying a trigger pulse to gate 29g.The power supply capacitor 31 is discharged through inductance 32 in aresonant manner such that it swings negative. This negative swing turnsoff crowbar SCR 29. Filter choke 33 is of relatively high inductance andkeeps current from the bias source 22 from increasing rapidly. As aresult, the negative voltage remains at the terminals of capacitor 31until capacitor 31 can be restored to its normal level E by current flowI through inductance 33 and through resistor 23' by way of the diode 21in the (RF) switch.

Since the voltage has gone negative, the SCRs 17 and 18 are back biasedand turned ofi. Since the SCRs will naturally turn off when the currentthrough them goes to zero, it is only necessary to remove the biasmomentarily until one of the pair turns off. The above crowbar turn offsystem operates by removing the bias for an interval in excess ofone-half the period of the (RF) cycle representing the lowest operatingfrequency. In One embodiment utilizing the circuit of this invention,turn ofi" occurred at the first (RF) zero crossing that followed an offtrigger. This 01? trigger was originally initiated by a negative going,trailing edge of a data pulse square wave applied to gate 29g of SCR 29.

The (RF) switch circuit of the invention may be broken into threesubsections: the switch section itself; drivers; and bias. In the switchsection, D.-C. current is caused to flow through the SCRs 17 and 18 froman external low voltage D.-C. source 22 to keep SCRs 17 and 18 on whenonce turned on. To turn the switch off, the D.-C. bias source is simplyopened. A circuit diagram of a complete switch assembly, with bias, inaccordance with the invention as used on a (VLF) antenna tuner, is shownin FIG. 7.

Bias supply involves basically an eifective means of bias control. Whenthe (RF) switch circuit is driven on by gate drive, bias current flowsand keeps the switch on. In order to turn the switch circuit 011? aresonant crowbar circuit is used. This circuit when triggered places alow inductance across the output capacitance of the power supply. Theresulting resonant discharge of the capacitor causes the etfective biasvoltage to reverse and stop the bias current through the SCRs 17" and18" for a time sufficient for them to recover by forced commutation fromthe (VLF) signal. Recovery of the power supply then reapplies biasvoltage to the switch in preparation for the next turn on signal.Functioning of the bias control is illustrated in FIGS. 5 and 6.

Use of a bias system as described above requires some form of turn-offdrive as Well as a turn-on drive. Turn-on of the switch SCRs 17" and 18is accomplished with a burst of gate pulses timed to occur with currentzeroes in the antenna current. A burst is used as insurance of tum-on inview of certain dynamic conditions which may occur in the system as aresult of multiple switch operations simultaneously. Turn-ofi isaccomplished by a single pulse applied to the gate 29'g of crowbar SCR29'.

The energy for removing the bias is derived from capacitor 31', which ischarged to essentially the same voltage as the bias source 22' bysampling the voltage drop across resistor 23" during the time the (RC)switch circuit is conducting. Since the capacitor voltage is of theopposite polarity to that of the bias source 22", whenever the crowbarSCR 29' conducts, the bias to the (RF) switch is removed bycancellation.

The coupling of a switching coil 35 to a helix (not shown) is preset, tothe amount of reactance change required, as the transmitter keyer shiftsthe frequency. As the angle of the switching coil 35 to the horizontaldecreases, coupling increases and, not only frequency shift (change ofresistance) increases, but, so does the (kva) across the switching coil35 and the SCRs. The additional circuit components not already discussedare for transient suppression and for rounding ofi sharp voltage edgeswhich might cause retriggering of the SCRs by dv/dt and lead to turn-offproblems.

To assist those skilled in the art in making and using a preferredembodiment of the invention, typical circuit values for the embodimentillustrated in FIG. 7 are set forth below. It will be understood,however, that such typical values are given by way of illustration onlyand in no sense by way of limitation. In general, stock components wereutilized, and the circuit can be made physically smaller and moreeconomical as specially sized or designed components are utilized.

SCRs 17" and 18" W254M. SCR 29' CZO-D Diodes 19" and 21" INll9OA. Diodes38 and 42 IN1348A Resistor 23" 5 ohms. Resistor 36 ohms. Resistor 41 5 Kohms. Capacitor 31' 8 ,uf. Capacitor 37 .01 #f. Capacitor 39 .47 of.Inductance 32 10 H. Inductance 33 1 ,uH.

Thus a switching circuit utilizing SCRs to modulate an antenna tuningcircuit by sequentially shorting or opening coils has been disclosed.The present invention utilizes a minimum number of components andemploys SCRs at their power frequency ratings to control signals at(VLF) frequencies.

It should be understood, of course, that the foregoing disclosurerelates to only preferred embodiments of the invention and that numerousmodifications or alterations may be made therein without departing fromthe spirit and the scope of the invention as set forth in the appendedclaims.

What is claimed is:

1. A circuit for controlling current flow through a load comprising:

first and second solid state controlled rectifiers each having an anode,a cathode and a gate control element, the anode of said first controlledrectifier and the cathode of said second controlled rectifier beingconnected to a first end of said load;

first and second diodes, each having an anode and a cathode, the cathodeof said first diode and the anode of said second diode being connectedto a second end of said load; and

control means connected at a first end to a junction of the cathode ofsaid first controlled rectifier and the anode of said first diode andconnected at a second end to a junction of the anode of said secondcontrolled rectifier and the cathode of said second diode, whereby D.-C.biasing is provided for maintaining conduction and an o signal fordiscontinuing conduction of said solid state controlled rectifiers.

2. The circuit of claim 1 wherein said control means comprises:

an external voltage source for providing a bias voltage;

a current limiting means connected to said voltage source for limitingthe flow of bias current; and

switch means connected in series with said external voltage source andsaid current limiting means and adapted for switching said solid statecontrolled rectifiers from their conducting to their nonconductingstate.

3. The circuit of claim 2 wherein said switch means comprises:

a mechanical make and break switch.

4. The circuit of claim 3 wherein said switch means comprises:

a resonant crowbar turn-off circuit having a solid state controlledrectifier, gated by an external data pulse for removing the bias currentand thereby switching said circuit to a nonconducting state.

5. A switching circuit for controlling current flow from an alternatingcurrent source through a load comprising:

a first subcircuit having a first electron discharge means and a firstdiode in series with each other and in series with said load andoperating in a first polarity with respect to an incoming signal;

a second subcircuit having a second electron discharge means and asecond diode in series with each other 7 8 and in series with said loadand in parallel relaan external voltage source adapted to provide a biastionship with said first subcircuit and operating in a current for saidcircuit; second polarity with respect to an incoming signal; currentlimiting means connected to said voltage source and for limiting theflow of bias current;

a third subcircuit having said first and second electron a resonantcrowbar turn-off circuit having a silicon discharge means in seriesrelationship with each other a controlled rectifier and gated by anexternal data and in series relationship with an external bias sourcepulse, said turn-off circuit adapted for removing and a switch means,said third subcircuit thereby prothe bias current and thereby switchingsaid circuit viding D.-C. biasing for maintaining conduction and an I0 3noncofldllcting State; and

ofi Signal for discontinuing conduction f Said first 10 reactance meansfor providing transient suppression and Second subcircuits and forrounding off sharp voltage edges thereby preventing undesirableretriggering of said silicon 6. The circuit of claim 5 wherein saidfirst and second controlled rectifiers.

electron discharge means are silicon controlled rectifiers. 7. A circuitfor controlling current fiow through a load com 15 References Citedprising.

first and second silicon controlled rectifiers each hav- UNITED STATESPATENTS ing an anode, a cathode and a gate control element, 3,277,36210/ 1966 Elliott 307-252 the anode of said first controlled rectifierand the 3,302,031 1/ 1967 Gutzwiller 307 252 cathode of said secondcontrolled rectifier being con- 9 connected to a first end of said load;N LD D. FORRER, Primary Examiner first and second diodes, each having ananode and J. DPREN, Assistant E i a cathode, the cathode of said firstdiode and the anode of said second diode being connected to a sec- U.S.Cl. X.R. ond end of said load; 2 307-305

